Tank Math Confusion

[UnixSage]

Consider the following info:
http://www.diveriteexpress.com/gas/steel.shtml#capacity

Now the last line (X8) 130cf for 3442psi
so 3442/130=26.47psi per cuft

So if I get a 3000psi fill I come up with
3000/26.47=113.33cuft

Why does the list say 117?

I am only off by 4cuft so it is not a big deal it just bugs me that I can not come up with the same number. If I work up a ratio (3000/3442)*130 I come up with the same number. Am I applying a linear relationship to something that does not have one?

Thanks
UnixSage

 

[Captain CaveMan]

http://www.spearfishing.org/bruces_tips/java/tankv.html

here you go

 

[Nostromo]

I would start by looking up "ideal gas laws" and how they apply to scuba. Hope that helps.

Consider the following info:
http://www.diveriteexpress.com/gas/steel.shtml#capacity

Now the last line (X8) 130cf for 3442psi
so 3442/130=26.47psi per cuft

So if I get a 3000psi fill I come up with
3000/26.47=113.33cuft

Why does the list say 117?

I am only off by 4cuft so it is not a big deal it just bugs me that I can not come up with the same number. If I work up a ratio (3000/3442)*130 I come up with the same number. Am I applying a linear relationship to something that does not have one?

Thanks
UnixSage

 

[UnixSage]

http://www.spearfishing.org/bruces_tips/java/tankv.html
here you go

Gives me the same number that I am comming up with.. Looking at the code on the website I see that it is using the exact same formula as I am

I would start by looking up "ideal gas laws" and how they apply to scuba. Hope that helps.

I have (or at least I think I did) a pretty good handle on that. Those formulas also deal with tempature but since I am working on the same chart I am assuming that all the gas is at the same tempature.

 

[UnixSage]

The good news is that the popular formula is close enough and also a little less so if any thing you will have more gas.

Thanks for the Info and education

 

[Spectre]

So if I get a 3000psi fill I come up with
3000/26.47=113.33cuft

Why does the list say 117?

It also says '104' for the 130s at 2640. their list makes some assumptions that aren't exactly true.

Stick with your math. you got it.

 

[Divin'Papaw]

I also come up with the same numbers you do.

 

[Dean810]

http://www.babilim.co.uk/pages/gas_laws.html. This site gives an interesting discussion of all of this, athough it uses different units (and has more math than then my poor brain can handle!). You are right that the math is not linear. You are also right that that your calculations are good enough most the time when dealing with air. HOWEVER, those using Trimix should be very careful. Using a linear formula would fool them into thinking they have much more air than they really do.

 

[UnixSage]

http://www.babilim.co.uk/pages/gas_laws.html

Thanks for the link.. Made for intersting reading. The fact the Helium makes that more of a delta is real interesting. Not using Trimix but it is good to keep in mind.